class FormValidationMessageOrderTest

class FormValidationMessageOrderTest extends KernelTestBase implements FormInterface {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_validation_error_message_order_test';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    // Prepare fields with weights specified.
    $form['one'] = [
      '#type' => 'textfield',
      '#title' => 'One',
      '#required' => TRUE,
      '#weight' => 40,
    ];
    $form['two'] = [
      '#type' => 'textfield',
      '#title' => 'Two',
      '#required' => TRUE,
      '#weight' => 30,
    ];
    $form['three'] = [
      '#type' => 'textfield',
      '#title' => 'Three',
      '#required' => TRUE,
      '#weight' => 10,
    ];
    $form['four'] = [
      '#type' => 'textfield',
      '#title' => 'Four',
      '#required' => TRUE,
      '#weight' => 20,
    ];
    $form['actions'] = [
      '#type' => 'actions',
      'submit' => [
        '#type' => 'submit',
        '#value' => 'Submit',
      ],
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  }
  
  /**
   * Tests that fields validation messages are sorted in the fields order.
   */
  public function testLimitValidationErrors() {
    $form_state = new FormState();
    $form_builder = $this->container
      ->get('form_builder');
    $form_builder->submitForm($this, $form_state);
    $messages = \Drupal::messenger()->all();
    \Drupal::messenger()->deleteAll();
    $this->assertTrue(isset($messages['error']));
    $error_messages = $messages['error'];
    $this->assertEquals('Three field is required.', $error_messages[0]);
    $this->assertEquals('Four field is required.', $error_messages[1]);
    $this->assertEquals('Two field is required.', $error_messages[2]);
    $this->assertEquals('One field is required.', $error_messages[3]);
  }

}